Thursday, 23 August 2007

The Lives Of Stars

I am currently reading Carl Sagan’s classic popular science book Cosmos, and felt inspired to write about a chapter I particularly enjoyed. Chapter IX, ‘The Lives of Stars’, takes us on a journey through space and time, looking at the Sun as well as some of its distant cousin stars, all of which behave in strange and wonderful ways.

Probably the most surprising thing about stars is that it all boils down to simple chemistry. Four hydrogen nuclei will combine, under very high gravitational pressure and temperature, to form a helium nucleus and emit light as a gamma ray photon. This is the almost disappointingly simple answer to a question that has tormented humankind since we first realized that there was actually a huge hot bright disc up there, a question that has led millions to invent all sorts of farfetched hypotheses and religions to explain it away. It really gets interesting when you travel backwards and forwards in time to see where it all comes from and where it will all end up. Let’s take it from the top, then.

The big bang was an explosion and rapid expansion of the fabric of spacetime, which consisted of some matter in the form of protons, neutrons and electrons, as well as a huge amount of nothing. The rapid cooling that followed due to this expansion caused these elementary particles to form hydrogen and helium gas clouds. The explosion itself was uneven, so clouds began to form clusters of various sizes, collapsing into themselves under the force of gravity. These massive clouds of gas are the birthplaces of millions of stars, eventually forming the galaxies we see and live in today, such as the Andromeda galaxy pictured on the right. Stars consist of that same gas having collapsed into itself at various points in space.

Stars are essentially massive engines that burn hydrogen. When temperatures in the core of a star are high enough (over 10 million degrees), the collapse stops as the outer layer is held back by the combustion taking place in the core. The photons emitted by the reaction take a million years to reach the outer layer. The sun has been a simultaneously exploding and collapsing hydrogen bomb for about 5 billion years, and it will continue to behave that way for about as long. Eventually, all engines run out of fuel, and so do all stars, but that does not always mean their death.

As the hydrogen runs out, the reaction will begin to cool and the star will expand outwards, engulfing the inner solar system. However, it will soon begin collapsing again under its own gravitational force, this time until temperatures get high enough to burn helium. Sagan compares this beautifully to a Phoenix rising out of its ashes, except this is not just an ancient myth but a real event that is constantly happening throughout the universe. The remaining hydrogen left over in the expanded region of the star will burn while helium burns at the core at higher temperatures. This is a red giant, with a hot carbon and oxygen-producing helium reactor in its core and a planet-engulfing hydrogen-burning outer region.

When the helium runs out, it does mean the end for most stars. A new expansion will take place, and the star will shoot out concentric shells of gas that will form the planetary nebula (pictured below). At this stage, the Sun would engulf Pluto. A few more massive stars can recollapse and burn carbon and oxygen for a while, but this is not very common. After the sun expands for the last time, the solar system will become a blue and red-fluorescent dead world. Billions of years later, the exposed core will become a white dwarf, and eventually a cold, dead black dwarf.

A planetary nebula

There are so many different aspects to this story that rival any storyteller’s wildest imagination. The poetic elegance of the lives of stars masks their terrible and devastating effect on the observing civilizations of their orbiting planets, but the universe is of course entirely indifferent and apathetic. I strongly recommend Cosmos to anyone who wants to catch a glimpse of the amazing things astronomy has discovered, especially since the invention of the radio telescope which can take us right to the edge of the universe.

Some stars do explode in supernovae, but only the ones that are massive enough to do so. They burn their hydrogen much faster than "normal" stars, then they explode into a supernova and leave behind a fast-spinning neutron star which we observe as a pulsar. Black holes are formed by the collapse of very massive stars or the collision between two neutron stars. And yes, stars are born inside nebulae like the well known horsehead nebula.

Contributors

We are geneticists and biochemists, alumni of the University of York (2009), now doing PhDs at the Universities of Cambridge, Leeds, Oxford and Vermont. We aim to bring to your attention interesting science, whether it is making headlines or not, referencing the original peer-reviewed research as often as possible.